To obtain an automatic nailing method for a siding and a nailer capable of being automatically nailed at specific positions by detecting the end of the siding, joint, etc., in free positions arranged on a furring strip.

In this automatic nailing method of a siding, a square siding 21 is mounted on a wall panel and is automatically nailed. In that case, two distance sensors SL and SR are arranged at a specific interval to the insides of two nailers arranged at a specific interval, furring strip 27, lower tongue, siding joint, and siding surface heights S1, S2, S3, and S4 are detected by two distance sensors SL and SR, and the heights to be detected are compared with each other to decide automatic nailing positions.